The objective of the majority of biological research today is to understand human diseases and develop new and effective treatments for them. Multidisciplinary research approaches serve to identify the genetic basis of disease in order to better understand and treat them. Due to the enormous complexity of cellular and molecular processes, as well as ethical issues associated with experiments on human subjects, researchers have focused on a number of eukaryotic systems that are simpler and easier to manipulate, yet complex enough to address many of the questions relevant to human biology. For instance, neurodegenerative diseases result from the progressive loss of neurons in the brain. The hallmark of these diseases is the accumulation of certain proteins inside the neuronal cells, leading to the loss of function, structure and, ultimately death of the cell. Finding treatments or cures for these diseases is challenging as very little is known about the underlying causes of these diseases.
Animal models offer an ideal system to observe the implications of complex interaction of the disease mechanism in the whole organism. The nematode, Caenorhabditis elegans (C. elegans), is one such model organism that has greatly facilitated the study of conserved biological processes. It offers a number of useful features such as small size (˜1,000 somatic cells), well-mapped neuronal connectivity, transparency, short life cycle (˜2.5 days), and the ability to generate many progeny in a relatively short time. Furthermore, the identity and lineage of every cell in the worm is known which enables researchers to address biological questions at single cell resolution. The amenability of the organism to genetic analysis has led to isolation of a large pool of mutant strains that can be effectively used to study gene function in regulating various developmental, behavioural, and physiological processes. Such studies have established the roles of many genes (e.g. HOX family members) and signalling pathways (e.g. Ras) in normal and disease processes, thereby facilitating the study of their homologues in other organisms including humans.
The analysis of the C. elegans genome sequence has revealed the presence of a large number (˜65%) of human disease orthologs that are very useful in investigating the underlying mechanism of gene function. Worms have been used successfully as models for a variety of human disorders such as obesity, hypertension, Duchenne's Muscular Dystrophy (DMD), and neurodegeneration (e.g., Huntington's disease, HD; Parkinson's disease, PD). Researchers have established mutant strains for these diseases that serve as models to study the underlying mechanism as well as to search for chemical compounds/drugs to inhibit the defects. For example, in the case of the C. elegans HD model, chemical screening has identified two compounds, mithramycin (MTR) and trichostatin A (TSA), that have significant effect on promoting neuronal survival. PD also results from the loss of dopamine neurons. PD patients show motor symptoms such as slow, imbalance body movement, stiffness of body (akinesia). Current treatments (such as levodopa, bromocriptine, cabergoline, lisuride and pergolide) provide only a temporary relief but cannot cure patients completely. This is because of the poor understanding of the etiology of PD. C. elegans PD models have been generated that show degeneration of DA neurons and movement defects. C. elegans has almost all the genes involved in Parkinson's disease, making it possible to study the genetic basis of the disease. Importantly, the mechanism for dopamine transportation and signalling are conserved between human and C. elegans. C. elegans has a simple dopamine neuron system consisting of only eight neurons; two pairs of CEPs, a pair of ADE in the head region and another pair of PDE in the mid body.
In addition to mutations in PD-associated genes, exposure of certain chemicals, such as 6-hydroxy dopamine (6-OHDA), methyl phenyl tetrahydro pyridine (MPTP) and rotenone (a pesticide), also induces PD-like symptoms in humans and other animal models, including C. elegans. These neurotoxins work, in part, by causing degeneration of DA neurons. 6-OHDA is also endogenously produced by the dopamine neurons as a byproduct of dopamine and inhibits the mitochondrial respiratory enzyme complex I and IV. MPP+ is the active toxic product of MPTP that inactivates the mitochondrial enzyme complex I of respiratory chain. Toxin exposed worms have been shown to serve as effective PD models to study the basis of neurodegeneration. These models also facilitate screening of genes and compounds that protect neurons from toxin-induced damage.
Among the various features of C. elegans, its small size and the ability to grow in liquid media have facilitated high throughput screenings (HTS) for chemicals. Chemicals which affect physiological processes, thus, may serve as potential drug candidates for a variety of medical applications. Conventional methods of chemical and animal screening involve exposure of a certain population of synchronized-age or -mutant model animals to thousands of chemical compounds individually and inside multi-well plate dishes, while monitoring the subsequent effects of the drugs on animals' growth, fertility, and other biological processes by immobilization and visual inspection. The above-mentioned methods are either manual and hence prone to human errors and time-consuming, or robotically automated and hence expensive and inaccessible to the majority of researchers. In addition, most conventional plate-based methods are currently focused on the cellular level analysis and tend to ignore the movement behaviour of C. elegans as one the most important parameters (especially for movement disorder diseases). Cellular level analysis is mostly done through immobilization and GFP imaging. Methods for immobilizing worms (anesthesia or gluing) are fatal, non-reversible and not suited for post-experimentation studies. The glue or anaesthetic chemical compositions' effect on C. elegans biological processes is also not well understood.
Microsystem engineering has played a critical role in providing the necessary technologies to tackle the challenges of small organisms' manipulation and analysis. C. elegans worms survive in liquid environment and due to their matching size scale with microfluidics (submicron to hundreds of micrometers), they have recently been studied in such devices. This has resulted in a dramatic increase in the experimental accuracy, consistency (by removing human interferences) and decrease in the cost of automation. Recently, microfluidic devices have been used for more precise and quantitative analysis of C. elegans development and behaviour. These devices have also been used for analyzing nematodes behaviour and mechanical characteristics in response to diverse stimuli, in-vivo imaging of their neuronal activity, culturing, sorting and screening, and in-vivo studies of neuronal regeneration after laser nano and micro-surgery on individual animals. These microfluidic devices have significantly facilitated assays on worms in an automated high throughput manner. In order to visualize and manipulate animals within these environments, their natural movement is eliminated by the use of hydraulic and pneumatic flows and forces. Accordingly, these devices are complicated in terms of fabrication (multilayers of PDMS microstructures aligned and bonded together) and operation (computer-controlled pneumatics). Also, despite being advantageous in phenotypic, cellular and sub-cellular studies, these devices are not suitable for performing movement-related behavioural studies on worms.